1. Field of the Invention
The present invention relates to a toner for developing electrostatic images. In addition, the present invention also relates to an image forming apparatus and a process cartridge using the toner.
2. Discussion of the Background
Electrophotographic image forming apparatuses such as copiers, printers, facsimiles and multi-function printers have been broadly used because of being capable of recording images on various kinds of receiving materials at high speed.
In order to fulfill a need for an image forming apparatus capable of producing high quality images at higher speed, development has been made in various fields (such as machine, toner and photoreceptor fields).
In order to fulfill the need for high quality images, for example, toner having a small average particle diameter and a spherical form has to be used. In order to fulfill the need for high speed recording, for example, toner having a better fixability has to be used. Specifically, a need has been existing for a toner which has a spherical from, a small average particle diameter, a sharp particle diameter distribution, a sharp charge quantity distribution, and a good fixability.
On the other hand, kneading/pulverization methods have been used for preparing toner. It is hard for such kneading/pulverization methods to produce toner having a small average particle diameter and to control the particle form of toner. In addition, since it is hard for such kneading/pulverization methods to use resins, which cannot be well pulverized, as binder resins of the toner, it is difficult to produce toner having a good combination of fixability and pulverizability by kneading/pulverization methods. Therefore, in attempting to solve the problem, other toner preparation methods such as emulsion aggregation methods, and suspension polymerization methods have been developed. By using such toner preparation methods, toner having a small average particle diameter, a sharp particle diameter distribution and a sharp charge quantity distribution can be efficiently prepared.
Recently, the need for high speed recording is growing more and more. Therefore, a need exists for toner having a better combination of fixability and durability. Specifically, when the image forming speed of an image forming apparatus (particularly, the developing speed of a developing speed thereof) is increased, the stresses applied to the toner in the developing device increases, thereby causing a problem (a cracking problem) in that the particles of the toner are cracked or disintegrated by the stresses, resulting in formation of fine toner particles and production of abnormal images. Therefore, a need exists for toner having a good durability so as not to produce fine toner particles even when used for a long period of time.
In attempting to impart good fixability to toner, techniques in that polyester resins are used as binder resins have been proposed. This is because polyester resins have lower melt viscosity and higher elasticity than other resins such as styrene-acrylic resins and therefore the resultant toner has a good combination of low temperature fixability and high temperature preservability.
In attempting to develop a toner fulfilling the above-mentioned needs, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2002-328489 discloses a toner preparation method in which a polymer prepared by using a monomer having a silicone atom or a fluorine atom and a polymer prepared by using a monomer having an acidic polar group are aggregated to prepare toner particles.
In addition, JP-A 2005-274615 discloses a toner in which the ratios of silicon atoms or fluorine atoms to carbon atoms in the surface portion and inner portion of the toner are specified so that the amount of silicon atoms or fluorine atoms included in the surface portion is greater than that in the inner portion. It is described therein that the surface of toner particles has a relatively low surface energy and therefore the toner can maintain a good reliability even when used for a long period of time.
However, as a result of the present inventors' investigation, it is found that the durability and reliability of these toners are not satisfactory for recent image forming apparatuses. In addition, it is also found that when emulsion aggregation methods are used and particles to be fused by heat to produce aggregated particles include a crosslinkable component, the crosslinkable component hinders fusion aggregation of the particles, and thereby the resultant toner particles have an unstable aggregated state. Therefore, when the toner is used, the above-mentioned cracking problem is caused. Thus, the long-term durability and reliability of a toner cannot be improved by merely decreasing the surface energy of the toner.
Because of these reasons, a need exists for a toner which has a good low-temperature fixability, and hardly causes an offset problem (in that a toner image is adhered to a fixing member, resulting in formation of abnormal images or jamming of a receiving sheet bearing an image thereon) even when used for image forming apparatus using an oil-less fixing device, or an image forming apparatus which can produce images at high speed without causing the offset problem.